The present invention relates to the production of valve metal powders of maximum capacitance useful in the production of valve metal electrodes, for instance, tantalum anodes, for electrolytic capacitors having especially high specific electric charges, by the use of sinter metallurgy at a sintering temperature which is below that customary according to the prior art. At the same time the physical and technical properties with respect to powder-metallurgical processing as well as the chemical purity of tantalum metal powder are improved. The invention also relates to the production of valve metal anodes having a high specific electric charge and improved electrical properties.
Especially tantalum and alloys thereof, but in general metals of the groups IVb, Vb and VIb of the Periodic Table and alloys thereof, may be considered as valve metals (see also "Oxides and Oxide Films", Vol. 1, published by John W. Diggle, pages 94 and 95, 1972, Marcel Dekker, Inc., New York). The complicated relations of production, treatment and electrical properties of valve metal powders are discussed below in the indicated prior art references, which refer to the use of tantalum.
It is state of the art that tantalum metal powder is compressed to porous shaped bodies with or without a binder or is otherwise formed, and after the removal of the binder is subjected to a temperature treatment under high vacuum. The purpose of this sintering process is to increase the strength of the shaped bodies for further post-treatments and also, by means of degassing (evaporation) or diffusion processes, to remove impurities which by nature are present in the metal powder, or which have been introduced during the process of production, so as to provide satisfactory electrical properties in capacitor anodes produced therefrom. Impurities, especially those of a metallic nature, considerably impair the electrical properties of a tantalum capacitor, above all the electric strength and the leakage current, as, for example, is apparent from U.S. Pat. No. 3,418,106, column 1, lines 58ff.
Moreover, non-metallic impurities, especially oxygen, nitrogen, silicon, etc., lead, on the one hand, to brittle fracture of the sintered tantalum feed wire, and, on the other hand, affect the sintering nature of tantalum powder compacts in that said impurities, as barriers between the powder particles, hinder the diffusion processes determined by the sintering. As known from the literature (Eisenkolb, Thummler, Grundlage der Pulvermetallurgie, Berlin: 1963), such surface effects are generally of importance for sintering processes. An insufficient sintering of a tantalum anode causes intermetallic conduction bridges between individual particles of the sintered compact to be too weak and, therefore, to some extent totally to be destroyed or to become inoperative when forming the dielectric oxide film with the final result that the geometrically available total surface of the tantalum anode is not fully effective. Besides, the resistance component of the capacitor impedance therefore become very high. Moreover, the development of very thin metallic conduction bridges, i.e. conductors with very small cross-sections, leads to local overheating during the forming process in consequence of high electric current density and, hence, leads to the formation of crystalline oxide zones which, contrary to an amorphous oxide, have a high electric conducting capacity and therefore considerably degrade the capacitance properties of a valve metal anode.
In order to attain sufficient sintering of the tantalum anode, one has to apply high sintering temperatures which according to the state of the art are not below 1600.degree. C., but mostly reach up to 2000.degree. C. (see U.S. Pat. No. 3,892,310, 7 Table 1A and column 8, line 14 ff; U.S. Pat. No. 3,299,326).
Moreover, the high sintering temperatures bring about a purifying effect in that impurities evaporate under high vacuum, or diffuse from the metal surfaces into a metal grind (grate). In this way pure surfaces are formed which, on the one hand, permit the sintering process to take place and, on the other hand guarantee the formation of trouble-free oxide films (dielectrics) when forming the anodes. High sintering temperatures, however, have the disadvantageous side effect that the available "active" surface is reduced while the surface-rich fine particles of the tantalum powder sinter with one another so that the porosity and surface of the sintered anode are altogether reduced. This effect becomes evident, for example, when comparing specific charges (mC/g) under changed sintering conditions (1600.degree.-1900.degree. C.) in FIG. 4 of U.S. Pat. No. 3,829,310.
To reduce the aforementioned surface losses by sintering, valve metal powders according to the prior art are "agglomerated", i.e. subjected to thermal pre-treatment in vacuo. This thermal pre-treatment, as, for example, described in U.S. Pat. Nos. 3,418,106 and 4,017,302, leads to powder aggregates consisting of fine particles, the surface of which is very much preserved over a wide temperature range when sintering valve metal anodes. Besides, this thermal agglomeration has a positive influence on the most important parameters of the powder-metallurgical treatment of valve metal, such as flowability, compressive stability and porosity of the anodes.
It is also true that limits should be set to the fineness of the powder particles of the indicated metal powders because the valve metal surfaces form a passivating oxide film under the influence of atmospheric oxygen. For that reason the oxide content of fine-grained metal powders is naturally higher than that of coarser grain fractions. This is also particularly true for agglomerated tantalum powders which are sintered together from expecially fine individual particles. In this case there takes place an additional absorption of oxygen after the thermal agglomeration process and in consequence of a reactivation of the metal surface connected therewith. The temperature of the agglomeration process usually lies clearly below those temperatures which are applied in the sintering of anodes. Of course, when proceeding from the present state of the art, the temperature range is limited toward lower temperatures for both the agglomeration of powders and the sintering of anodes.
In principle, it would have been advantageous to sinter, e.g., tantalum, powder to porous capacitor anodes at as low temperatures as possible, i.e. within a temperature range below 1600.degree. C., had it not been necessary to observe the heretofore pointed out pre-conditions according to the prior art.